D. c. to d. c. regulated converter



Dec. 27, 1966 R. P. MASSEY D.C. TO D.C. REGULATED CONVERTER OriginalFiled March 5, 1963 /Nl/E/\/7O/? By R. P. MASSEY 5 M ATTORA/EV UnitedStates Patent 3,295,043 D.C. T0 DC. REGULATED CONVERTER Richard P.Massey, Westfieid, N.J., assignor to Eel! T elephone Laboratories,lncorporatcd, New York, N.Y., a

corporation of New York Continuation of application Ser. No. 263,662,Mar. 5,

1963. This application Feb. 15, 1966, Ser. No. 537,591

3 Claims. (Cl. 321-2) This application is a continuation of applicationSerial No. 263,062, which was filed March 5, 1963, now abandoned.

The invention relates generally to electrical current supply circuitsand more particularly to electrical current supply circuits which areregulated to maintain substantially constant output voltage.

The usual voltage regulator operates with a closed feedback loop. Anerror detector compares the output or load voltage with a standardreference voltage and feeds any discrepancy back to a regulating devicewhich is located between the input voltage source and the errordetector. The regulating device is normally a variable impedance deviceor a switch and is connected either in series with or in shunt acrossthe input voltage source. Such regulators are versatile in that theycorrect not only for changes in input voltage but also for loadimpedance variations, but tend to be relatively complex and can besomewhat wasteful of power. Because they normally provide no D.C.isolation between input source and load, such regulators can also betroubled by unwanted ground loops and a resulting instability and shockhazard.

One object of the present invention is to increase the eificiency ofvoltage regulation in an electrical current supply circuit in as simplea manner as possible.

Another object is to increase the efficiency of voltage regulation and,at the same time, provide effective D.C. isolation between the input andoutput circuits of the regulator.

The invention takes advantage of the fact that there are frequentoccasions when a voltage regulator need correct only for changes ininput voltage and makes use of a so-callcd DC. to DC. converter toprovide regulation on an open-loop basis. While the resulting regulatordoes not compensate for changes in load impedance, it has increasedefiiciency, greater simplicity, and provides complete D.C. isolationbetween input and output circuits.

In the electrical current supply art, a converter is a circuit fortransforming a direct current at one voltage level to another directcurrent at a different voltage level and is normally made up of aninverter for changing the direct input current to an alternatingcurrent, a transformer (which may be the output or feedback transformerof the inverter) for changing the voltage level of the alternatingcurrent, and an output rectifier. Such circuits are generally quitesimple, waste very little power, and provide good D.C. isolation betweeninput and output circuits. They provide no regulation, however, sincechanges in input voltage result in corresponding changes in output orload voltage.

In accordance with the invention, a D.C. to DC converter is made tofunction as an open-loop regulator by providing the output or feedbacktransformer of its inverter with a saturable core having a substantiallyrectangular hysteresis loop and driving the inverter in such a manner asto maintain its operating frequency substantially constant. Thesaturable transformer core maintains the volt-time product of the AC.wave generated by the inverter substantially constant, making themagnitude of the output voltage derived therefrom substantiallyindependent of variations in input voltage magnitude. The open-loopregulator retains, of course, all of 3,295,043 Patented Dec. 27, 1966the usual advantages of a DC. to DO. converter in that it is relativelysimple, wastes very little power, and provides good D.C. isolation. Itcan be used, moreover, to provide either a step-up or a step-down involtage. Because there is no closed regulating feedback loop, anylikelihood of unwanted ground loops, with the attendant chance of shockhazard or instability, is eliminated.

In a preferred embodiment of the invention, the inverter (called thepower inverter for reference convenience) of the DC. to DC. converter isdriven at a constant frequency by another inverter. The driving inverteris powered from the same input voltage source as the power inverter buthas its operating frequency specially stabilized against variations ininput voltage magnitude. The constant-frequency operation of the powerinverter permits the saturable core of its output or feedbacktransformer to perform the regulating function by maintaining a constantvolt-time product in the manner described.

A more complete understanding of the invention may be obtained from astudy of the following detailed description of the specific embodimentillustrated in the drawing.

In the illustrated embodiment of the invention, the power inverter isdriven at a constant frequency fixed by an auxiliary inverter poweredfrom the same input voltage source but stabilized in frequency againstchanges in input voltage magnitude in the manner disclosed and claimedin applicants United States Patent 3,215,952, which issued November 2,1965.

The frequency-stabilized driving inverter is in the form of afour-terminal bridge circuit in which a pair of pnp transistors 11 and12 form one pair of adjacent arms and a pair of voltage-dividingcapacitors 13 and 14 the other. As illustrated, the emitter-collectorpaths of transistors 11 and 12 are connected in series with one another,with the emitter of transistor 11 and the collector of transistor 12forming one of the four terminals of the bridge. To apply a direct inputvoltage across one diagonal of the bridge, the terminal formed bycapacitor 14 and the emitter of transistor 12 is grounded and the oneformed by capacitor 13 and the collector of transistor 11 is connectedto the negative side of an input D.C. source 15 through a smoothingfilter consisting of a shunt capacitor 16 and a series inductor 17. Inaddition, a germanium diode 18 is connected across inductor 17 toeliminate any voltage transients created by turning the converter on andoff. The output of the frequency-stabilized inverter is taken from atransformer 21, one winding 22 of which is connected from the bridgeterminal between capacitors 13 and 14 to that formed by the emitter oftransistor 11 and the collector of transistor 12.

Regenerative feedback is provided in the driving inverter by twoadditional windings 23 and 24 of transformer 21. The relative polaritiesof the windings are as indicated by the dots. One end of winding 23 isconnected to the emitter of transistor 11, while the other is connectedthrough a timing capacitor 25 and a timing resistor 26 to the base oftransistor 11. One end of winding 24, on the other hand, is grounded (asis the emitter of transistor 12) while the other is connected through atiming capacitor 27 and a timing resistor 28 to the base of transistor12. Timing resistors 26 and 28 are variable and ganged, as illustrated,to provide fine control of the driving inverter operating frequency.Finally, a diode 29 is connected between the base and emitter electrodesof transistor 11 and poled toward the latter electrode, while a diode 30is similarly connected and poled between the base and emitter electrodesof transistor 12.

In operation, transistors 11 and 12 of the driving inverter conductalternately. The voltage division between capacitors 13 and 14 issubstantially equal. When tra11- sistor 12 is conducting (insaturation), half of the input voltage is applied to winding 22 andinduces voltages on the other windings of transformer 21. The voltage onwinding 22 is negative at the dot, as are those on windings 23 and 24.The negative potential applied to timing capacitor 27 by winding 24holds transistor 12 in its conducting state, while the positivepotential applied to capacitor 25 by winding 23 and developed acrossdiode 29 holds transistor 11 in its non-conducting state. As capacitor27 charges, the base current of transistor 12 decreases exponentially.When it reaches the value [1 I is the collector current flowing in thetransistor and ,8 is the transistor common-emitter current gain,transistor 12 comes out of saturation and the voltage across itsemitter-collector path increases. At the same time, the reverse bias ontransistor 11 decreases as capacitor 25 charges. By regenerative action,transistor 11 switches to its conducting (saturated) state andtransistor 12 switches to its non-conducting state. The voltage acrosswinding 22 of transformer 21 reverses and the induced voltages acrossall of the windings of the transformer 21 also reverse. The cyclerepeats itself, maintaining oscillation, with the frequency determinedprimarily by the time constant of capacitor 25 and resistor 26 and bythe time constant of capacitor 27 and resistor 28.

The frequency of the driving inverter is stabilized against inputvoltage variations by a pair of diode-resistor networks connected toinject into the base of each transistor when conducting a reversecurrent at least equal to the maximum 1 for that transistor, where 1 isthe transistor collector current which still flows in the absence ofemitter current. One of these networks, made up of the serialcombination of a current-limiting resistor 31 and a diode 32, isconnected from the end of transformer winding 24 indicated by the dot tothe base of transistor 11, The other, consisting of the serialcombination of a current-limiting resistor 33 and a diode 34, isconnected from the end of winding 23 remote from the dot to the base oftransistor 12. Diodes 32 and 34 are both poled toward the baseelectrodes of their re spective transistors and resistors 31 and 33 bothcooperate with their respective transformer windings to fix the injectedcurrents at at least the maximum values of 1 for their respectivetransistors. The injected currents are preferably limited to values onlyslightly in excess of the maximum values of 1 for the transistors inorder to provide complete frequency stability without unnecessary lossof power.

In accordance with an important feature of the invention, thefrequency-stabilized inverter which has just been described is employedto drive the main or power inverter of a D.C. to D.C. converter. Theoutput or feedback transformer of the power inverter is, in accordancewith another important feature of the invention, provided with asaturable core having a substantially rectangular hysteresis loop. Thepower inverter in the illustrated embodiment of the invention is withoutindependent frequency stabilization, but is of the same general type asa driving inverter. It takes the form of another four-terminal bridgecircuit in which a pair of p-n-p transistors 35 and 36 form one pair ofadjacent arms and voltage-dividing capacitors 13 and 14 form the other.A pair of diodes 37 and 38 are connected in series with one anotheracross capacitors 13 and 14 to assist in maintaining exact voltagedivision across the capacitor voltage divider. As illustrated, diodes 37and 38 are poled from D.C. source toward ground so that they are biasedin the reverse direction.

The output from the power inverter is taken from the bridge diagonalbetween the terminal formed by the respective emitter and collectorelectrodes of transistors 35 and 36 and the terminal formed betweencapacitors 13 and 14. Half of a centertapped winding 39 of an outputtransformer 40 is connected across that diagonal.

The other half of winding 39 is connected, as illustrated, from thejunction of capacitors 13 and 14 to the junction between diodes 37 and38. Exact voltage division across the capacitor voltage divider isobtained through the use of this portion of winding 39 in conjunctionwith diodes 37 and 38. If the voltage across capacitor 14 tends to risewhile transistor 36 is conducting, due to an unbalance in the capacitiesof the two capacitors, the voltage across the upper portion of winding39 also tends to rise. The voltage across the lower portion of thewinding rises at the same time. Since the resulting voltage across thelower portion of winding 39 is greater than the voltage across capacitor13, the ensuing current through diode 37 charges capacitor 13 andrestores equal voltage division. Similarly, if the voltage acrosscapacitor 13 tends to exceed that across capacitor 14, diode 38 conductsand restores equal voltage division.

Regenerative feedback in the power inverter is provided by a pair offeedback windings 41 and 42 of transformer 411. As illustrated, winding41 is connected to the base of transistor 35, while winding 42 isconnected to that of transistor 36. To provide accurate control of thefrequency of the power inverter, output transformer 21 of the drivinginverter, which is stabilized in frequency against variations in inputvoltage magnitude, is equipped with a pair of output windings 43 and 44.One end of winding 43 is connected through a capacitor 45 and acurrent-limiting resistor 46 to the end of winding 41 remote fromtransistor 35. Resistor 46 is shunted by a capacitor 47. The other endof winding 43 is connected to the bridge terminal formed by the junctionbetween the emitter of transistor 35 and the collector of transistor 36.One end of winding 44, on the other hand, is connected through acapacitor 48 and a currentlimiting resistor 49 to the end of winding 42remote from transistor 36. Resistor 49 is shunted by a capacitor Stl.The other end of winding 44 is grounded. The relative polarities of allof the windings of transformers 21 and 40 are as indicated by the dots.Finally, a first Zener diode 55 is connected between the emitter andbase electrodes of transistor 35 and poled in the forward directiontoward the base, while a second Zener diode 56 is similarly connectedand poled between the emitter and base electrodes of transistor 36.

The power inverter in the illustrated embodiment of the inventionoperates in both frequency and phase synchronism with the drivinginverter. Transistors 11 and 35 conduct in phase with one another, as dotransistors 12 and 36. When transistors 12 and 36 are conducting, thevoltages across windings 43 and 44 of transformer 21 are negative at thedots and series additive with those across windings 41 and 42 oftransformer 40. The combined voltage across windings 42 and 44 forwardbiases Zener diode 5'6 and the base-emitter junction of transistor 36,driving transistor 36 into full conduction. This base driving current,the bulk of which flows through transistor 36 rather than through diode56, is limited by resistor 49 and charges capacitors 4-8 and 50. At thesame time, the combined voltage across windings 41 and 43 breaks downZener diode 55 in the reverse direction, charging capacitors 45 and 47through current-limiting resistor 46. Since Zener diode 55 is inbreakdown, it provides a reverse cut-off voltage for transistor 35 andholds it in its non-conducting state.

As has already been indicated, the core of transformer 40 is of therectangular hysteresis loop variety. For this reason, it saturatesbefore the base currents in the driving inverter transistors decrease tothe switching point. When the core of transformer 40 saturates, there isno further change in flux and all winding voltages fall to zero. Thedifference between the voltage across capacitor 48 and that acrosswinding 44 of transformer 21 is positive and small enough not to breakdown Zener diode 56. Also, the sum of the voltage across capacitor 45and that across winding 43 of transformer 21 is posi tive and smallenough not to break down Zener diode 55. Since Zener diodes 55 and 56are reverse biased, they are effective open circuits, causing bothtransistors 35 and 36 of the power inverter to be non-conducting.

When transistors 11 and 12 of the driving inverter switch so thattransistor 11 conducts and transistor 12 is shut off, the voltagesacross windings 43 and 44 of transformer 21 reverse polarity. The sum ofthe voltages across capacitors 48 and 50 and the voltage across winding44 of transformer 21 is positive and high enough to break down Zenerdiode 56. Transistor 36 is thus held in its non-conducting state. At thesame time, the difference between the voltage across capacitor 45 andthe voltage across windings 43 of transformer 21 is negative, forwardbiasing Zener diode 55 and the base-emitter junction of transistor 35.Transistor 35 is thereby driven into conduction. The voltage on theupper portion of winding 39 of transformer 40 then reverses polarity andthe cycle repeats. Capacitors 47 and 50 help decrease the turn-off timesof power inverter transistors 35 and 36.

The output voltage of the power inverter in the illustrated embodimentof the invention appears at an output winding 61 of saturabletransformer 40 as alternate halves of a square wave separated by zerovoltage intervals. Full-wave rectification is provided by a pair ofdiodes 62 and 63, each connected to an opposite end of output winding61. The rectified voltage is filtered by passing it through a seriesinductor 64 to the load 65. A shunt capacitor 66 between inductor 64 andload 65 aids in the filtering. A bleeder resistor 67 is shunted acrossload 65 to limit output voltage in the event of an open-circuited load.

Line voltage regulation without a closed feedback loop in theillustrated embodiment of the invent-ion is achieved by the use ofmaterial having a substantially rectangular hysteresis loop for the coreof transformer 40. Such a material has a constant volt-time product. Forthis reason, transformer 40 saturates earlier in the cycle when thevoltage applied to the upper portion of winding 39 increases and laterwhen that voltage decreases. Since there is no further change in fluxwhen transformer 40 saturates, all winding voltages fall to zero,switching the conducting one of power inverter transistors 35 and 35 toits nonconducting state. Since the other transistor is not switched toits conducting state until the corresponding transistor in the drivinginverter begins to conduct, the over-all period of the output waveappearing on output winding 61 of transformer 40 consists of oneconduction interval and one zero voltage interval of transistor 36followed by one conduction interval and one zero voltage interval oftransistor 35. If the input voltage supplied by DC. source increases,the conduction intervals shorten and the zero voltage intervals lengthenby a corresponding amount. If the input voltage decreases, on the otherhand, the conduction intervals lengthen and the zero voltage intervalsshorten. Since the over-all period of the power inverter output wave is,in accordance with an important feature of the invention, held constantby the driving inverter even though the input voltage changes, theaverage value of the rectified ouput of the power inverter is heldconstant. The final filtered output voltage appearing across load 65 isthereby made independent of input voltage variations and a regulatingeffect is achieved without the use of a closed feedback loop,

Because there is no power dissipation in a series or shunt regulatingelement, the illustrated regulator approaches optimum efficiency. It isextremely simple and, for that reason, rugged and highly reliable. TheDC. to DC. converter permits either a step-up or a step-down in voltageand the lack of a closed feedback loop permits complete -D.C. isolationof the input and output circuits.

The illustrated regulator may, in addition, be operated as a closed-loopregulator in order to achieve closer load voltage regulation than isobtainable from an ordinary closed-loop regulator. A conventional errordetector may be used to detect departures of the load voltage from areference value and the resulting error signal used to vary a resistanceplaced across an additional winding of trans former 21. Resistancevariations are reflected back into the driving inverter and adjust itsfrequency in such a way as to regulate the load voltage.

It is to be understood that the above-described arrangement isillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:

1. A regulated converter which comprises a source of direct inputvoltage, an output rectifier, a separately excited inverter powered bysaid source connected to supply an alternating voltage to said outputrectifier, said separately excited inverter including a pair oftransistors, a pair of capacitors, the emitter-collector paths of saidtransistors being connected in series to form two adjacent arms of afour-terminal bridge and said capacitors being connected in series toform the remaining two arms of the same bridge, and a saturable outputtransformer having a primary winding connected across the diagonal ofsaid bridge formed by the juncture between said transistors and thejuncture between said capacitors, a pair of feedback windings eachconnected to the base electrode of a respective one of said transistorsto render the emittercollector paths of said transistors alternatelyconducting and non-conducting in phase opposition to each other, and asecondary winding connected to said output rectifier, said source beingconnected across the other diagonal of said bridge and said saturableoutput transformer having a substantially rectangular hysteresis loopand saturating at least once during every half cycle of said alternatingvoltage, and a substantially constant frequency self-excited inverterpowered by said source connected with its output coupled to saidfeedback windings to drive said separately excited inverter at saidsubstantially constant frequency, whereby said saturable transformermaintains the volttime product of said alternating voltage substantiallyconstant and the magnitude of the direct voltage derived therefrom bysaid output rectifier is independent of variations in the magnitude ofsaid direct input voltage.

2. A regulated converter in accordance with claim 1 in which saidself-excited inverter includes said pair of capacitors and a second pairof transistors, the emittercollector path of said second pair oftransistors being connected in series to form two adjacent arms of asecond four-terminal bridge and said capacitors also forming theremaining arms of said second bridge.

3. A regulated converter in accordance with claim 2 in which meansabstracting the output from said self-excited inverter is connectedacross the diagonal of said second bridge formed by the juncture betweensaid second pair of transistors and the juncture between said capacitorsand said source is connected across the other diagonal of said secondbridge.

References Cited by the Examiner UNITED STATES PATENTS 2,987,665 6/1961Thompson 321-16 3,219,906 11/1965 Keller et al 321-2 3,219,907 11/1965Josephson 321-2 X References Cited by the Applicant UNITED STATESPATENTS 3,117,270 1/1964 Tailleur.

JOHN F. COUCH, Primary Examiner. W. H. BEHA, Assistant Examiner.

1. A REGULATED CONVERTER WHICH COMPRISES A SOURCE OF DIRECT INPUTVOLTAGE, AN OUTPUT RECTIFIER, A SEPARATELY EXCITED INVERTER POWERED BYSAID SOURCE CONNECTED TO SUPPLY AN ALTERNATING VOLTAGE TO SAID OUTPUTRECTIFIER, SAID SEPARATELY EXCITED INVERTER INCLUDING A PAIR OFTRANSISTORS, A PAIR OF CAPACITORS, THE EMITTER-COLLECTOR PATHS OF SAIDTRANSISTORS BEING CONNECTED IN SERIES TO FORM TWO ADJACENT ARMS OF AFOUR-TERMINAL BRIDGE AND SAID CAPACITORS BEING CONNECTED IN SERIES TOFORM THE REMAINING TWO ARMS OF THE SAME BRIDGE, AND A SATURABLE OUTPUTTRANSFORMER HAVING A PRIMARY WINDING CONNECTED ACROSS THE DIAGONAL OFSAID BRIDGE FORMED BY THE JUNCTURE BETWEEN SAID TRANSISTORS AND THEJUNCTURE BETWEEN SAID CAPACITORS, A PAIR OF FEEDBACK WINDINGS EACHCONNECTED TO THE BASE ELECTRODE OF A RESPECTIVE ONE OF SAID TRANSISTORSTO RENDER THE EMITTERCOLLECTOR PATHS OF SAID TRANSISTORS ALTERNATELYCONDUCTING AND NON-CONDUCTING IN PHASE OPPOSITION TO EACH OTHER, AND ASECONDARY WINDING CONNECTED TO SAID OUTPUT RECTIFIER, SAID SOURCE BEINGCONNECTED ACROSS THE OTHER DIAGONAL OF